The radiating web of troughs seen emanating from a crater that lies in the bulls-eye of Mercury’s giant Caloris basin was formed by an asteroid impact, say scientists presenting their research at the European Planetary Science Congress today.

Could the radiating spiderweb-like trough features be associated with the impact crater in the centre of the Caloris basin? MESSENGER's principal investigator thinks so. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The troughs are hundreds of kilometres in length and appear to radiate from the 40 kilometre wide central crater, which has now been named Apollodorus, after the architect of the Pantheon temple in Rome. The feature was first observed in MESSENGER’s maiden flyby of Mercury at the start of this year, and much debate surrounding the nature and origin of the feature has followed. MESSENGER revealed that the Caloris basin appeared to be flooded with ancient, now solidified volcanic lavas, similar to the mare seen on the Moon, but the discovery of ‘the spider’ is unlike any other structure seen in lunar basins or elsewhere on Mercury.

Several models have been proposed for the trough’s formation, including uplift of the basin due to heating from below, pressure building up from the superposition of surrounding plains or movements in the crust. However, to date, none of these models could explain the radial pattern. To help shed some light on the mystery, MESSENGER’s Principal Investigator Dr Sean Solomon and colleagues have developed a three-dimensional model of deformations in Mercury’s crust and then modelled the effect of an impact at the centre.

“We found that stresses building up within the crust could explain the troughs found around the circumference of the basin but not the radial web at the centre,” he says. “When we modelled the effect of a meteorite striking the centre of a pre-stressed basin floor, we found that the formation of the crater relieved the stress build-up and weakened the central area, allowing the troughs to spread out like cracks in a windscreen."

MESSENGER's first flyby revealed volcanism as a key player in the formation of Mercury's plains. In this false-colour image, the orange stain highlights the giant 2,300 metre wide Caloris basin, in which 'the spider' resides. Image:NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington.

The scientists conclude that as the central crater appears to be superimposed over the troughs, then the network of troughs must have formed simultaneously with crater. However, not all scientists agree that the crater’s presence at the centre of the web is anything more than coincidence.

Professor Jim Head of Brown University, and co-investigator of the MESSENGER mission, believes that the troughs could instead have resulted from volcanic activity. Upflow of magma from a deep reservoir underneath the centre of the Caloris basin could have formed a bulge in the surface that later collapsed into a radial network of fractures. Alternatively the troughs could have resulted from fresh lavas breaking apart as they cooled and contracted.

“The first MESSENGER flyby provided a lot of evidence that volcanism has played an important role in Mercury’s history, in particular around the Caloris Basin,” he says. “We found what appears to be a shield volcano located just outside the Caloris Basin and the area is surrounded by smooth plains, relatively free from impacts, which suggests a young surface. Given the amount of volcanic activity we’re discovering in that area, I wouldn’t want to rule out a volcanic cause just yet.”

The scientists will have to wait for MESSENGER’s upcoming second flyby, scheduled for 6 October, to settle the debate.

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